Book value is the difference between a company's assets and liabilities. The lower the price-book value, the cheaper the valuation.

However, this does not mean you should only invest with the expectation that this measure will immediately revert to its mean.

In fact, this measure (and many other valuation measures) will drift from long-term averages for long periods of time. It's further proof that markets will be irrational longer than you can stay solvent.